It is known in the prior art to self test a MEMS device, such as during a quality control phase of a fabrication process. Some MEMS devices, such as accelerometers, include spring-supported movable masses. In a defective unit, the mass may be immobilized, such as by debris trapped in a space into which the mass is design to move, or the spring may be too stiff or not stiff enough, or the mass may not be sufficiently attached to surrounding structures. In either case, in response to accelerating the device, the mass may not move as it was designed to do. For example, the mass may move too much, too little or not at all.
During testing, an electrical signal is typically applied to the device to move a movable mass, or several movable masses, in a given direction, typically the same direction as acceleration is normally sensed by the device, and any movement of the mass or masses is measured. If the mass or masses do not move as expected, a defect is detected.
Such self testing may be adversely affected by ambient motion or noise. Such corruption is usually compensated for by averaging measurements over a sufficiently long time, such that the test signal can be recovered to a desired accuracy. Alternatively, efforts can be made to shield the device from such ambient disturbances during testing. However, such additional test time or sensor shielding during manufacturing adds to the cost of the product.
An alternate approach to making self testing accurate, despite ambient disturbances, has been to utilize a test signal having a frequencies higher than the expected ambient rumble or a device's frequency response. In such cases, the outputs can be high-pass filtered to obtain the results of the test. With this approach, the test signal response would, however, be at a significantly lower output level. Furthermore, the results would be subject to high frequency resonances.